1. Field of the Invention
The present invention relates to the sealing and cutting of hollow slabs, for example concrete slabs, having one or more cores.
2. Description of the Prior Art
Continuous pre-stressed hollow core concrete slabs and their formation are described, for example, in document GB-A-2,256,380. Such slabs may be made in a variety of ways such as by the extrusion, sheer compaction or slip forming methods and this is typically done on pre-stressing beds of approximately 100 meters length. The substantially uniform slabs are formed to this length with hollow cores running longitudinally. Once the concrete of a continuous slab has cured, smaller individual slabs of a predetermined length are cut from the long one. The individual slabs are then transported to building sites or to wherever else they may be required and are, for example, used in forming floors of buildings.
During the cutting of a larger slab into individual slabs a mechanically operated sawing device is used and the saw blade of this device is cooled by the use of high pressure water. Large quantities of the water combine with the concrete dust from the slabs during the cutting process, and form a fine slurry which frequently enters the cores running perpendicular to the cut. The slurry dries there and remains in the bottoms of the cores of the individual slabs at a distance of up to 1.5 meters either side of the cut.
If the cores are not used at any later stage, this slurry is of no importance. On the other hand, if the cores of the individual slabs are to be utilised at a later stage, for example to transfer air to ventilate rooms, it is necessary to clean the cores thoroughly by removing all the dried slurry residue and any other debris which may have accumulated in the cores en route to their destination, during storage or during installation. Cleaning the cores can be an expensive and time consuming operation. Further, once a core has been cleaned, it is usual to block it, preferably at both ends, to prevent any further debris accumulating in it. If the use to which the cores are to be put, is, for example, transferring air, then the blocks at the ends should be air-tight. Holes will then be provided in the undersides or top surfaces of the slabs and ducted air enters and exits the cores at precisely determined locations. It should not leak haphazardly through the slab ends.
Conventionally, concrete mortar plugs are inserted into the ends of the cores in order to seal them. To ensure that the wet concrete does not go down the cores during the sealing operation and effectively block them up, a further, non-liquid stopper must be introduced beforehand into the cores. Such stoppers are usually made of hard insulation plastic, previously fabricated to the correct design size of the cores. The disadvantage of this is that in practice, in the manufactured product, the core sizes and the shapes of cores in a hollow core slab vary considerably from each other and from their theoretical sizes and shapes. Thus the stoppers may not fit because they are either the wrong shape or size, and will allow concrete to seep past them, particularly whilst the concrete is being compacted. Further, during compaction, the pressure of the flowing concrete has a tendency to force the stopper further down into the core. It can be difficult to prevent this by fixing the stoppers into the cores securely.
This method also gives no guarantee that, even with the concrete restrained against the core stopper, there is an air-tight seal at the ends of the cores. This necessitates a specific air pressure test on each core, together with a time consuming smoke test to detect leakages from the core should they occur.
These processes are hampered by the fact that the cleaning and plugging operations are generally carried out at the building site and usually once the slabs have been installed, or immediately prior to installation in their correct locations. The whole operation as described is therefore linked to the critical path of the general construction site program and because of the laborious nature of the operation, it can be extremely time consuming with the result that site work can be delayed.